Electrostatic-capacity-type acceleration sensor and acceleration measuring device therewith

ABSTRACT

A moving member having a plurality of moving electrodes is supported by support members at both ends thereof on a substrate surface in such a way that it can be subjected to displacement in a two-dimensional plane. A plurality of fixed electrodes are arranged to face the plurality of moving electrodes respectively, thus forming different facing areas therebetween when an input acceleration is zero. The facing areas formed between pairs of the electrodes facing each other are varied in response to the displacement of the moving member, whereby a capacitance caused by one pair of the electrodes whose facing area is relatively small is used to detect a relatively small input acceleration, and a capacitance caused by the other pair of the electrodes whose facing area is relatively large is used to detect a relatively large input acceleration.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 10/790,440,filed Mar. 1, 2004 now U.S. Pat. No. 7,004,027, by Toshiyuki NOMURA,which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrostatic-capacity-type accelerationsensors that detect accelerations based on variations in electrostaticcapacity (or capacitance), and it also relates to acceleration measuringdevices using electrostatic-capacity-type acceleration sensors.

This application claims priority on Japanese Patent Application No.2003-55562, the content of which is incorporated herein by reference.

2. Description of the Related Art

Conventionally, a typical type of an electrostatic-capacity-typeacceleration sensor (as disclosed in Japanese Patent ApplicationPublication No. Hei 7-260510) comprises a capacitance detector having amoving electrode and a fixed electrode, which are arranged opposite toeach other such that a separating distance therebetween is varied inresponse to an input acceleration, wherein a switched-capacitor circuitconverts capacitance variation due to displacement of the movingelectrode into voltage variation so as to detect and output anacceleration signal, and wherein in order to broaden the detectionrange, a servo circuit is provided so as to hold the moving electrode ata neutral position by feeding back the acceleration signal.

Another type of the electrostatic-capacity-type acceleration sensor (asdisclosed in Japanese Patent Application Publication No. Hei 10-206457)comprises a moving electrode and a fixed electrode, which are arrangedon a substrate surface such that a facing area therebetween is varied inresponse to an input acceleration, wherein based on the capacitancerealized by the moving electrode and fixed electrode, the capacitancevariation due to the displacement of the moving electrode is detected soas to produce an acceleration signal.

The acceleration sensor having the servo circuit makes it possible tobroaden the detection range compared with the acceleration sensor notequipped with the servo circuit because the movement of the movingelectrode is limited in response to the output of the servo circuit.However, the range of the acceleration realized by the servo circuitstabilizing the moving electrode is limited by the electrostaticattraction and the weight of the moving electrode; hence, it is not easyto broaden the detection range.

The acceleration sensor in which the facing area between the movingelectrode and fixed electrode is varied provides only a single kind ofvariation characteristic with regard to the facing area responding tothe input acceleration; hence, it is not easy to broaden the detectionrange.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a brand-newelectrostatic-capacity-type acceleration sensor whose detection rangecan be broadened with ease.

An electrostatic-capacity-type acceleration sensor of this inventioncomprises a plurality of capacitance detectors having pairs of movingelectrodes and fixed electrodes that are arranged to face each other onthe substrate surface, wherein facing areas are varied in response to aninput acceleration. Herein, the capacitance detectors are constitutedsuch that the facing areas between the moving electrodes and fixedelectrodes differ from each other when the input acceleration is zero,so that each of the capacitance detectors produces an accelerationsignal in response to the capacitance variation occurring between thepaired electrodes thereof.

Since the acceleration sensor employs the aforementioned constitutionadapted in each of the capacitance detectors in which the facing areasbetween the moving electrodes and fixed electrodes differ from eachother so as to produce a variety of acceleration signals in response tocapacitance variations occurring between the paired electrodes, it ispossible to easily broaden the overall detection range for detecting theinput acceleration by simply increasing the number of the capacitancedetectors installed therein, wherein it is possible to provide eachcapacitance detector with a specific detection range suited thereto,thus realizing a high accuracy in the detection of the acceleration.

In the above, all of the moving electrodes provided for the capacitancedetectors can be integrally formed together. Thus, it is possible toimprove the integration of components and parts in the manufacturing ofthe acceleration sensor on the substrate surface by using the modernsemiconductor manufacturing technology.

Specifically, each of the capacitance detectors comprises two pairs ofmoving electrodes and fixed electrodes, which are arranged in such a waythat the facing area between the first moving electrode and first fixedelectrode increases while the facing area between the second movingelectrode and second fixed electrode decreases in response to the sameinput acceleration, whereby it produces an acceleration signal inresponse to a ratio or a difference between the first capacitanceactualized between the first moving electrode and first fixed electrodeand the second capacitance actualized between the second movingelectrode and second fixed electrode. Thus, it is possible to noticeablyimprove the detection sensitivity in each of the capacitance detectorsbecause the acceleration signal can be precisely produced in response tothe ratio or difference between the two types of the capacitance.

In addition, the capacitance detectors can be constituted such that onecapacitance detector has a relatively small facing area between thepaired electrodes so as to produce an acceleration signal in response toa relatively small input acceleration, and the other capacitancedetector has a relatively large facing area between the pairedelectrodes so as to produce an acceleration signal in response to arelatively large input acceleration. Thus, it is possible to detect theinput acceleration with a high sensitivity in a relatively broaddetection range.

An acceleration measuring device of this invention comprises anelectrostatic-capacity-type acceleration sensor including first andsecond capacitance detectors each having a pair of a moving electrodeand a fixed electrode in which the facing area between the pairedelectrodes in the second capacitance detector is set to be larger thanthe facing area between the paired electrodes in the first capacitancedetector when an input acceleration is zero, a first detection circuitfor producing a first acceleration signal in response to capacitancevariation occurring between the moving electrode and fixed electrode inthe first capacitance detector, a second detection circuit for producinga second acceleration signal in response to capacitance variationoccurring between the moving electrode and fixed electrode in the secondcapacitance detector, and a selector for selectively outputting thefirst acceleration signal as long as the first acceleration signal doesnot exceed a threshold level determined in advance and for selectivelyoutputting the second acceleration signal when the first accelerationsignal exceeds the threshold level.

The aforementioned acceleration measuring device can measureacceleration at a high accuracy in a relatively broad detection range.The threshold value set to the selector can be determined to exclude asharp varying region in a capacitance varying characteristic establishedwith respect to variations of the input acceleration; hence, it ispossible to reliably avoid occurrence of error detection due to noise.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, aspects, and embodiments of the presentinvention will be described in more detail with reference to thefollowing drawings, in which:

FIG. 1A shows a schematic arrangement of electrodes in a capacitancedetector in which facing areas between moving electrodes and fixedelectrodes are decreased in response to acceleration;

FIG. 1B shows a schematic arrangement of electrodes in a capacitancedetector in which facing areas between moving electrodes and fixedelectrodes are increased in response to acceleration;

FIG. 2 is a graph showing curves representing relationships between theinput acceleration and capacitance varying ratio (C_(U)/C_(D)) withrespect to various electrode overlap values (L);

FIG. 3 is a plan view showing a layout of parts constituting anelectrostatic-capacity-type acceleration sensor in accordance with apreferred embodiment of the invention; and

FIG. 4 is a block diagram showing an example of an accelerationmeasuring circuit using the electrostatic-capacity-type accelerationsensor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention will be described in further detail by way of exampleswith reference to the accompanying drawings.

FIGS. 1A and 1B show examples of arrangements of electrodes incapacitance detectors used for an electrostatic-capacity-typeacceleration sensor in accordance with the preferred embodiment of theinvention, wherein FIG. 1A shows an arrangement of electrodes in thefacing area decreasing side, and FIG. 1B shows an arrangement ofelectrodes in the facing area increasing side. With reference to FIGS.1A and 1B, the operating principle of the electrostatic-capacity-typeacceleration sensor will be described.

In FIGS. 1A and 1B, each of moving electrodes M₁ to M₄ has a rectangularshape in which the length is set to ‘2L’ and the width is set to acertain value. Herein, the moving electrodes M₁ and M₃ are arrangedalong a dashed line L₁ in parallel with a prescribed distancetherebetween such that the centers in their length directions match thedashed line L₁. Similarly, the moving electrodes M₂ and M₄ are arrangedalong a dashed line L₂ in parallel with a prescribed distancetherebetween such that the centers in their length directions match thedashed line L₂. The dashed lines L₁ and L₂ are drawn in parallel witheach other. In FIG. 1A, the moving electrodes M₁ and M₂ are arranged inparallel with an electrode pitch ‘4L’ therebetween. In FIG. 1B, themoving electrodes M₃ and M₄ are arranged in parallel with an electrodepitch ‘4L’ therebetween. Each of the moving electrodes M₁ to M₄ has adeadweight function, so that they can be collectively subjected todisplacement in left-right directions with respect to the drawing sheetof FIGS. 1A and 1B in response to an input acceleration whilemaintaining the aforementioned relationship of arrangement.

In FIG. 1A, fixed electrodes S₁ and S₂ are respectively arranged underthe moving electrodes M₁ and M₂, wherein the fixed electrode S₁ isarranged to oppositely face the left-half portion (whose length is ‘L’)of the moving electrode M₁, and the fixed electrode S₂ is arranged tooppositely face the left-half portion (whose length is ‘L’) of themoving electrode M₂. For the sake of convenience, the fixed electrodesS₁ and S₂ are drawn not to overlap with the moving electrodes M₁ and M₂in FIG. 1A. Similarly, in FIG. 1B, fixed electrodes S₃ and S₄ arerespectively arranged under the moving electrodes M₃ and M₄, wherein thefixed electrode S₃ is arranged to oppositely face the right-half portion(whose length is ‘L’) of the moving electrode M₃, and the fixedelectrode S₄ is arranged to oppositely face the right-half portion(whose length is ‘L’) of the moving electrode M₄. For the sake ofconvenience, the fixed electrodes S₃ and S₄ are drawn not to overlapwith the moving electrodes M₃ and M₄ in FIG. 1B.

In the above, there are provided four pairs of the fixed electrodes andthe moving electrodes oppositely facing each other, namely, M₁-S₁,M₂-S₂, M₃-S₃, and M₄-S₄, each pair of which realizes a certaincapacitance. At 0 G where the input acceleration is zero, each pairrealizes the same capacitance ‘C₀’. Next, suppose that the inputacceleration of 1 G is applied on the moving electrodes M₁ to M₄, all ofwhich are moved rightwards as indicated by arrows in FIGS. 1A and 1B,and each of which is thus subjected to displacement in a distance ‘d’,the “paired” electrodes (i.e., M₁-S₁, and M₂-S₂) decrease the facingareas therebetween so that their capacitance decrease from C₀ to C_(D)in FIG. 1A, while the other “paired” electrodes (i.e., M₃-S₃, and M₄-S₄)increase the facing areas therebetween so that their capacitancesincrease from C₀ to C_(U). Therefore, when the input acceleration actingon the moving electrodes M₁ to M₄ becomes greater than 1G, thecapacitance C_(D) further decreases, and the capacitance C_(U) furtherincreases.

FIG. 2 is a graph showing relationships between the input accelerationand the capacitance varying ratio C_(U)/C_(D) with respect to variouselectrode overlap values, which are related to the electrode facing area‘L’. That is, the graph of FIG. 2 provides eight curves K₁, K₂, K₃, K₄,K₅, K₆, K₇, and K₈, each of which shows the relationship between theinput acceleration and the capacitance varying ratio C_(U)/C_(D) withregard to the electrode overlap value L, which is set to 0.5 μm, 0.75μm, 1.0 μm, 1.25 μm, 1.5 μm, 2.0 μm, 2.5 μm, and 5.0 μm respectively.

According to the graph of FIG. 2, as the electrode overlap value Lbecomes greater, it becomes possible to detect ‘greater’ inputacceleration. For this reason, the acceleration sensor of the presentembodiment comprises a plurality of capacitance detectors havingdifferent electrode overlap values, each of which is adequatelyconstituted to detect input acceleration in a certain detection rangesuited thereto. That is, a relatively small input acceleration can bedetected using a capacitance detector whose electrode overlap value isrelatively small, and a relatively large input acceleration is detectedusing a capacitance detector whose electrode overlap value is relativelylarge. Hence, it is possible to actualize a high sensitivity indetecting acceleration in a relatively broad range of detection.

With respect to each of the aforementioned curves (e.g., K1), in orderto avoid error detection due to noise, it is preferable not to use acertain region in which the capacitance varying ratio C_(U)/C_(D)becomes large so as to make variation sharp.

FIG. 3 shows an electrostatic-capacity-type acceleration sensor 10 inaccordance with the preferred embodiment of the invention.

In the acceleration sensor 10, a moving member MB having a deadweightfunction is arranged on the surface of a silicon substrate and issupported by four support members H₁ to H₄ at both sides thereof suchthat it can be subjected to displacement in a direction DS in parallelwith the substrate surface. The ends of the support members H₁ to H₄ arefixed at prescribed positions by fixing members P₁ to P₄ respectively.Four moving electrodes M₁₁, M₂₁, M₂₄, and M₁₄ are arranged on one sideof the moving member MB and are projected in parallel with the substratesurface. In addition, four moving electrodes M₁₃, M₂₃, M₂₂, and M₁₂ arearranged on the other side of the moving member MB and are projected inparallel with the substrate surface. All of the moving electrodes M₁₁ toM₁₄ have the same length projecting from the moving member MB, and allof the moving electrodes M₂₁ to M₂₄ have the same length projecting fromthe moving member MB. In addition, all of the moving electrodes M₁₁ toM₁₄ have the same width, and all of the moving electrodes M₂₁ to M₂₄have the same width that is greater than the width of the movingelectrodes M₁₁ to M₁₄.

All of the moving member MB, the moving electrodes M₁₁-M₁₄ and M₂₁-M₂₄,and the support members H₁-H₄ are integrally combined together as asingle assembly. Such an assembly can be formed by patterning conductivelayers, which are made of a semiconductor or a metal and are depositedon an insulating film covering the substrate surface, wherein after thepatterning, the insulating film is removed to allow movements of themoving member MB, the moving electrodes M₁₁-M₁₄ and M₂₁-M₂₄, and thesupport members H₁-H₄, for example. In addition, four fixing holes areformed at four fixing positions on the insulating film covering thesubstrate surface and are filled with conductive plugs, made of asemiconductor or a metal, thus forming the four fixing members P₁ to P₄.

Four fixed electrodes S₁₁ to S₁₄ are arranged opposite to the movingelectrodes M₁₁ to M₁₄ with relatively small facing areas therebetween,thus actualizing capacitances C₁₁ to C₁₄ having substantially the samevalue when the input acceleration is zero. In addition, four fixedelectrodes S₂₁ to S₂₄ are arranged opposite to the moving electrodes M₂₁to M₂₄ with relatively large facing areas therebetween, thus actualizingcapacitances C₂₁ to C₂₄ having substantially the same value when theinput acceleration is zero. Suppose that the moving member MB issubjected to displacement in the direction DS as indicated by an arrowin FIG. 4 (i.e., a forward direction in parallel with the sheet of FIG.4), wherein the fixed electrodes S₁₁, S₁₂, S₂₁, and S₂₂ are associatedwith the moving electrodes M₁₁, M₁₂, M₂₁, and M₂₂ such that the facingareas therebetween are reduced so as to decrease the capacitances C₁₁,C₁₂, C₂₁, and C₂₂ respectively, while the fixed electrodes S₁₃, S₁₄,S₂₃, and S₂₄ are associated with the moving electrodes M₁₃, M₁₄, M₂₃,and M₂₄ such that the facing areas therebetween are increased so as toincrease the capacitances C₁₃, C₁₄, C₂₃, and C₂₄ respectively. In FIG.4, the capacitances C₁₁-C₁₄ and C₂₁-C₂₄ are associated with arrows,wherein a downward slanted arrow indicates that the correspondingcapacitance is decreased, and an upward slanted arrow indicates that thecorresponding capacitance is increased.

The aforementioned fixed electrodes S₁₁-S₁₄ and S₂₁-S₂₄ are formed byimpurity-doped regions in which conductive impurities are selectivelydoped onto the substrate surface. Alternatively, they are formed bypatterning conductive layers, made of a semiconductor or a metal, whichare deposited on an insulating film covering the substrate surface.

Wiring layers W₁₁-W₁₄ and W₂₁-W₂₄ are respectively extended from thefixed electrodes S₁₁-S₁₄ and S₂₁-S₂₄. The wiring layers W₁₁ and W₁₂ areinterconnected with a detection line Ta via connection members Q₁₁ andQ₁₂ respectively. The wiring layers W₁₃ and W₁₄ are interconnected witha detection line Tb via connection members Q₁₃ and Q₁₄ respectively. Thewiring layers W₂₁ and W₂₂ are interconnected with a detection line Tcvia connection members Q₂₁ and Q₂₂ respectively. The wiring layers W₂₃and W₂₄ are interconnected with a detection line Td via connectionmembers Q₂₃ and Q₂₄ respectively. The support member H₁ is connectedwith a detection line Te.

The wiring layers W₁₁-W₁₄ and W₂₁-W₂₄ are formed by patterning such thattheir widths are reduced to be as small as possible in order to reduceparasitic capacitances thereof; the distances with the moving parts(i.e., MB, M₁₁-M₁₄, M₂₁-M₂₄) are increased to be as large as possible;and the electrode overlap values associated with the moving parts arereduced to be as small as possible. Similar to the fixed electrodesS₁₁-S₁₄ and S₂₁-S₂₄, the wiring layers W₁₁-W₁₄ and W₂₁-W₂₄ are formed byimpurity-doped regions or conductive layers subjected to patterning.Eight connection holes are formed at prescribed positions on theinsulating film covering the substrate surface and are filled withconductive plug, made of a semiconductor or a metal, thus actualizingthe connection members Q₁₁-Q₁₄ and Q₂₁-Q₂₄. Incidentally, the connectionmembers (e.g., Q₁₁) can be formed by adopting the aforementionedtreatment for use in the formation of the fixing members (e.g., P₁).

For example, after the wiring layers W₁₁-W₁₄ and W₂₁-W₂₄ are formed asimpurity-doped regions, a first wiring layer forming process isperformed on the insulating layer covering the substrate surface so asto form the fixed electrodes S₁₁-S₁₄ and S₂₁-S₂₄; then, a second wiringlayer forming process is performed on the insulating film covering thefixed electrodes S₁₁-S₁₄ and S₂₁-S₂₄ so as to form the moving member MB,the moving electrodes M₁₁-M₁₄ and M₂₁-M₂₄, the support members H₁-H₄,and the detection lines Ta-Td. Thus, it is possible to increasedistances between the wiring layers W₁₁-W₁₄ and W₂₁-W₂₄ and the movingparts (i.e., MB, M₁₁-M₁₄, M₂₁-M₂₄); hence, it becomes possible to reduceparasitice capacitances.

FIG. 4 shows an equivalent circuit of the acceleration sensor 10 shownin FIG. 3; specifically, it shows an example of an accelerationmeasuring circuit using the acceleration sensor 10 shown in FIG. 3. Theacceleration measuring circuit of FIG. 4 is designed to measure theinput acceleration in a range between 0 G and 8 G as shown in FIG. 2,wherein the curve K₇ is used for the measurement of a relatively smallinput acceleration that ranges from 0 G to 4 G and the curve K₈ is usedfor the measurement of a relatively large input acceleration that rangesfrom 4 G to 8 G Specifically, in a range R₁ wherein 0 G ≦inputacceleration≦4 G, compared with the curve K₈, the curve K₇ provides agreater value of the capacitance varying ratio C_(U)/C_(D), which inturn actualizes a high accuracy in measurement. In a range R₂ where 4G<input acceleration≦8 G, the curve K₇ includes a measurement-incapableregion and is not used in measurement, whereas the curve K₈ is suitablein measurement. In order to avoid error detection due to noise, it isnecessary not to use a sharp varying region of the curve K₇ with respectto the range R₁, and it is necessary not to use a sharp varying regionof the curve K₈ with respect to the great input acceleration over 8 G,wherein these regions are excluded from the measurement.

A detection circuit 12A is connected with the detection lines Ta, Tb,and Te; and a detection circuit 12B is connected with the detectionlines Tc, Td, and Te. Suppose that the capacitance varyingcharacteristic of the capacitance detector, including the capacitancesC₁₁ to C₁₄, against the input acceleration is shown by the curve K₇shown in FIG. 2, while the capacitance varying characteristic of thecapacitance detector, including the capacitances C₂₁ to C₂₄, against theinput acceleration is shown by the curve K₈ shown in FIG. 2, forexample. Herein, the capacitance varying ratio C_(U)/C_(D) is calculatedusing C₁₁-C₁₄ and C₂₁-C₂₄ as follows:

$\frac{C_{13} + C_{14}}{C_{11} + C_{12}}$$\frac{C_{23} + C_{24}}{C_{21} + C_{22}}$

With reference to the capacitance varying characteristic of the curveK₇, the detection circuit 12A produces an acceleration signal AS₁,representing the voltage related to a ratio (or a difference) between(C₁₃+C₁₄) and (C₁₁+C₁₂). With reference to the capacitance varyingcharacteristic of the curve K₈, the detection circuit 12B produces anacceleration signal AS₂ representing the voltage related to a ratio (ora difference) between (C₂₃+C₂₄) and (C₂₁+C₂₂). The acceleration signalAS₁ is increased in level as the input acceleration increases from 0 G.The acceleration signal AS₂ is set such that it substantially matchesthe acceleration signal AS₁ in level at the input acceleration of 4 G,and it is further increased from that level as the input accelerationincreases from 4 G.

A selector 14 has terminals A and B for receiving signals as well as aselect terminal SB. Herein, the selector 14 selectively outputs theacceleration signal SA₁ when SB is low (or at ‘0’), while it selectivelyoutputs a signal SO₂, which is output from a selector 18, when SB ishigh (or at ‘1’). A comparator 16 compares the acceleration signal AS₁,with reference voltage V_(R1) so as to produce a comparison output CO₁,which is supplied to the select terminal SB of the selector 14. That is,the comparator 16 outputs CO₁=0 when AS₁≦V_(R1); and it outputs CO₁=1when AS₁>V_(R1).

The prescribed voltage related to the capacitance varying ratioC_(U)/C_(D), which is read from the curve K₇ in FIG. 2 when the inputacceleration is 4 G is given as the reference voltage V_(R1) supplied tothe comparator 16. As a result, the selector 14 provides a selectionoutput SO₁, corresponding to the acceleration signal AS₁, which isproduced in response to the input acceleration in the range R₁ where 0G≦input acceleration≦4 G.

The selector 18 has terminals A and B for receiving signals as well as aselect terminal SB. That is, the selector 18 selectively outputs theacceleration signal SA₂ when SB is low (or at ‘0’), while it selectivelyoutputs an out-of-range signal ASx representing that the inputacceleration is out of the measurement range when SB is high (or at‘1’). A comparator 20 compares the acceleration signal AS₂ with areference voltage V_(R2) so as to provide a comparison output CO₂ to theselect terminal SB of the selector 18. That is, the comparator 20outputs CO₂=0 when AS₂≦V_(R2); and it outputs CO₂=1 when AS₂>V_(R2). Theprescribed voltage related to the capacitance varying ratio C_(U)/C_(D),which is read from the curve K₈ in FIG. 2 when the input acceleration is8 G is given as the reference voltage V_(R2) supplied to the comparator20. As a result, the selector 18 provides a selection output SO₂corresponding to the acceleration signal AS₂, which is produced inresponse to the input acceleration in the range R₂ where 4 G<inputacceleration≦8 G In this case, the selector 14 is set so as to selectthe terminal B for receiving the selection output SO₂ of the selector 18in response to the comparison output CO₁ of the comparator 16; hence,the selector 14 provides the selection output SO₁ corresponding to theacceleration signal AS₂.

When the acceleration signal AS₂ exceeds the reference voltage V_(R2)(where AS₂>V_(R2)), the comparison output CO₂ of the comparator 20 turnsto ‘1’; hence, the selector 18 provides the selection output SO₂corresponding to the out-of-range signal ASx. In this case, the selector14 is set so as to select the terminal B for receiving the selectionoutput SO₂ of the selector 18; hence, it provides the selection outputSO₁ corresponding to the out-of-range signal Asx. An accelerationdisplay (not shown) is provided to display an acceleration based on theacceleration signal AS₁ or AS₂ and to also display a message that theinput acceleration is out of the measurement range in response to theout-of-range signal ASx.

The aforementioned acceleration measuring circuit of FIG. 4 is designedto perform measurement at a high accuracy with respect to a relativelybroad range of 0 G ≦input acceleration≦8 G, which is realized by thecombination of the aforementioned ranges R₁ and R₂. Herein, themeasurement regarding the input acceleration is performed by excludingsharp varying regions of the curves K₇ and K₈ shown in FIG. 2; hence, itis possible to reliably avoid occurrence of error detection due tonoise. The acceleration measuring circuit of FIG. 4 is designedselectively using two curves K₇ and K₈ within the eight curves K₁ to K₈shown in FIG. 2. Of course, it is possible to easily modify theacceleration measuring circuit using three or more curves.

As described heretofore, an acceleration sensor of this invention isdesigned such that a plurality of capacitance detectors including pairsof moving electrodes and fixed electrodes in which facing areastherebetween differ from each other when the input acceleration is zeroare arranged on the substrate surface, wherein each of the capacitancedetectors is set to a prescribed measurement range of acceleration.Hence, by simply increasing the number of capacitance detectors, it ispossible to actualize broadening of the overall detection range ofacceleration for an acceleration measuring circuit; thus, it is possibleto realize acceleration detection at a high sensitivity and in arelatively broad range.

As this invention may be embodied in several forms without departingfrom the spirit or essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within metesand bounds of the claims, or equivalents of such metes and bounds aretherefore intended to be embraced by the claims.

1. An electrostatic-capacity-type acceleration sensor comprising: aplurality of capacitance detectors including plural pairs of movingelectrodes and fixed electrodes which are arranged to face with eachother so that facing areas therebetween are respectively varied inresponse to an input acceleration, the plurality of capacitancedetectors are constituted in such a way that under no acceleration thefacing area formed between one pair of the moving electrode and thefixed electrode differs from the facing area formed between other pairof the moving electrode and the fixed electrode, wherein an accelerationsignal is produced in response to capacitance variation caused by thepairs of the moving electrodes and the fixed electrodes.
 2. Theelectrostatic-capacity-type acceleration sensor according to claim 1,wherein a plurality of the moving electrodes are integrally formedtogether with respect to the plurality of capacitance detectors.
 3. Theelectrostatic-capacity-type acceleration sensor according to claim 1,wherein a relatively small input acceleration is detected based on theacceleration signal produced by one capacitance detector in which thefacing area between the moving electrode and the fixed electrode isrelatively small, while a relatively large input acceleration isdetected based on the acceleration signal produced by other capacitancedetector in which the facing area between the moving electrode and thefixed electrode is relatively large.
 4. The electrostatic-capacity-typeacceleration sensor according to claim 1, wherein each of thecapacitance detectors comprises first and second moving electrodes, andfirst and second fixed electrodes, which are respectively arranged toface with each other with first and second facing areas therebetween insuch a way that the first facing area formed between the first movingelectrode and the first fixed electrode decreases while the secondfacing area formed between the second moving electrode and the secondfixed electrode increases in response to the input acceleration, wherebythe capacitance detector produces the acceleration signal in response toa ratio or a difference between a first capacitance occurring betweenthe first moving electrode and the first fixed electrode and a secondcapacitance occurring between the second moving electrode and the secondfixed electrode.
 5. The electrostatic-capacity-type acceleration sensoraccording to claim 4, wherein the first and second moving electrodes areintegrally formed together with respect to the plurality of capacitancedetectors.
 6. The electrostatic-capacity-type acceleration sensoraccording to claim 4, wherein a relatively small input acceleration isdetected based on the acceleration signal produced by one capacitancedetector in which the facing area between the moving electrode and thefixed electrode is relatively small, while a relatively large inputacceleration is detected based on the acceleration signal produced byother capacitance detector in which the facing area between the movingelectrode and the fixed electrode is relatively large.